In recent years, liquid crystal display devices characterized by low power consumption, thinness, and light weight have been used as display devices of a TV, a personal computer, a mobile phone, etc. Such a liquid crystal display device does not emit light by itself, which is a so-called non-luminous display device, and therefore is designed to direct light from an illumination device (backlight) that is provided, e.g., on the back (non-display surface) side of a liquid crystal display element or ambient light onto the liquid crystal display element as illumination light. In the liquid crystal display device, the intensity of the illumination light is modulated by controlling the transmittance or reflectance of the liquid crystal display element in accordance with image signals so as to display images.
The above backlight is broadly divided into a direct type and a sidelight type depending on the location of a light source with respect to the liquid crystal display element. For the direct-type backlight, a plurality of light sources are located directly on the back side of the liquid crystal display element, and a diffuser or a focusing prism sheet is interposed between the light sources and the liquid crystal display element, thereby allowing uniform illumination light to enter the liquid crystal display element.
On the other hand, the sidelight-type backlight includes a light source located on the side of the liquid crystal display element and a light guide located with its side facing the light source. Light from the light source is introduced into the light guide directly through the side or via a reflector. In this sidelight-type backlight, while the light introduced into the light guide is propagated by total reflection inside the light guide, the light from the light source is emitted to the liquid crystal display element as illumination light appropriately by using patterns or the like provided on the surface, back or inside of the light guide. Moreover, the sidelight-type backlight generally includes several diffusion sheets or focusing prism sheets to improve the uniformity of light emitted from the light guide.
The above patterns of the sidelight-type backlight may be provided in the following manner: a resin for diffusing light is printed on the surface of the light guide; concave and convex portions for disusing light are formed in the surface of the light guide; particles for diffusing light are filled into the light guide; or prism-shaped structures are integrally molded with the light guide. The sidelight-type backlight can emit uniform planar light by forming the patterns so that the density varies with distance from the light source.
The direct-type backlight has the advantages of being able to achieve light weight and high brightness easily. Therefore, the direct-type backlight is used as an illumination device for a liquid crystal television with a relatively large screen size or a vehicle-mounted liquid crystal display device. For portable equipment with a relatively small screen size such as a mobile phone, PDA, or notebook PC, the sidelight-type backlight is mainly used because of its advantages of relatively low power consumption, thin, lightweight, and high brightness uniformity.
There has been a demand to further reduce the thickness and weight of the liquid crystal display device. However, it is difficult for the existing direct-type backlight and sidelight-type backlight, which requires a light guide and diffusion sheets or focusing prism sheets, to meet this demand. In the conventional liquid crystal display device, therefore, it has been proposed that a transparent substrate is used as one of a pair of substrates sandwiching a liquid crystal layer included in the liquid crystal display element, and light from the light source is introduced into the transparent substrate, so that the light guide can be removed (see, e.g., JP 9 (1997)-5739 A).
A liquid crystal display device of a first conventional example disclosed in JP 9 (1997)-5739 A will be described specifically with reference to FIG. 11.
As shown in FIG. 11, the liquid crystal display device of the first conventional example includes a liquid crystal layer 31, a pair of transparent substrates 32a, 32b sandwiching the liquid crystal layer 31, and a light guiding sheet 33 attached to the opposite surface of the transparent substrate 32a from the liquid crystal layer 31. Alight source 35 and a reflector 36 are located on the left side of the transparent substrate 32a and the light guiding sheet 33, and light from the light source 35 is introduced into the transparent substrate 32a and the light guiding sheet 33. The transparent substrates 32a, 32b are made of a transparent glass material having a refractive index of 1.60. The light guiding sheet 33 has a refractive index of 1.62 to 1.65 higher than that of the transparent substrate 32a. 
In the light guiding sheet 33, two or more types of transparent amorphous layers are stacked at a predetermined angle. The light incident from the light source 35 onto the transparent substrate 32a is reflected by an interface 34 between the two amorphous layers with different refractive indexes, and directed toward a first polarizer 37 and a reflector 38 that are located in this order on the lower side of the light guiding sheet 33. Moreover, of s-polarized light and p-polarized light contained in the light from the light source 35, the light guiding sheet 33 emits mainly the s-polarized light selectively to the reflector 38.
In the liquid crystal display device of the first conventional example with the above configuration, the light introduced from the light source 35 into each of the transparent substrate 32a and the light guiding sheet 33 is propagated through the light guiding sheet 33 in the direction away from the light source 35 while being totally reflected by utilizing the refractive index difference between the light guiding sheet 33 and the air and the refractive index difference between the light guiding sheet 33 and the transparent substrate 32a. When the total reflection conditions of the light traveling in the light guiding sheet 33 are not fulfilled by the presence of the interfaces 34 in the light guiding sheet 33, the s-polarized light is emitted to the reflector 38, and then is reflected from the reflector 38 to the light guiding sheet 33. Subsequently, the light reflected back to the light guiding sheet 33 enters the liquid crystal display element as illumination light, and passes through a second polarizer 39 provided on the upper surface of the transparent substrate 32b, so that images are displayed with the liquid crystal display element.
However, in this liquid crystal display device of the first conventional example, the light from the light source 35 is mainly propagated through the light guiding sheet 33. Therefore, to use the light from the light source 35 efficiently, the thickness of the light guiding sheet 33 should be at least about the same as that of the light source 35. Thus, the advantages of reduced thickness and weight may be impaired compared to other conventional liquid crystal display devices including the light guide. Although the transparent substrates 32a, 32b themselves can be made thinner to reduce the thickness and weight of the liquid crystal display device, the use of a thinner glass material for each of the transparent substrates 32a, 32b may cause new problems such as breakage of the glass material (cracking, glass chipping, etc.) during the manufacturing process. Accordingly, there is a limit to the reduction in thickness and weight of the liquid crystal display device of the first conventional example.
Moreover, in this liquid crystal display device of the first conventional example, the light from the light source 35 is extracted at each of the interfaces 34 in the light guiding sheet 33 and used as illumination light. However, since the reflectance of the interfaces 34 is very low, the light traveling in the light guiding sheet 33 cannot be efficiently emitted as illumination light.
In particular, when a light absorbing layer with relatively high light absorption properties such as a color filter or metal wiring is provided in the liquid crystal display element, the amount of illumination light absorbed by the light absorbing layer is increased because the refractive index difference between the transparent substrate 32a and the light guiding sheet 33 is small (0.02 to 0.05). This may lead to a significant reduction in the light utilization efficiency of the light source 35.
In the conventional liquid crystal display device, therefore, it has been proposed that a low refractive index layer is disposed on one side of the transparent substrate serving as the light guide that faces the liquid crystal layer, and an optical path control layer is disposed on the other side of the transparent substrate that faces away from the liquid crystal layer. This configuration is intended to reduce the thickness and weight of the liquid crystal display device and also to prevent a reduction in the light utilization efficiency (see, e.g., JP 2001-318379 A and JP 2002-121050 A).
Specifically, as shown in FIG. 12, a liquid crystal display device of a second conventional example disclosed in JP 2001-318379 A includes a liquid crystal layer 49, a pair of transparent substrates 44a, 44b sandwiching the liquid crystal layer 49, and a light source 51 located opposite to the side of the transparent substrate 44a. Light from the light source 51 is introduced into the transparent substrate 44a directly and via a reflector. The liquid crystal display device of the second conventional example is a reflection-type liquid crystal display device in which reflected light from a reflector 50 provided on the transparent substrate 44b is allowed to enter the liquid crystal display element as illumination light.
The transparent substrates 44a, 44b are made of a transparent glass material. A transparent low refractive index layer 45 having a lower refractive index than the transparent substrate 44a is disposed on the surface of the transparent substrate 44a that faces the liquid crystal layer 49. Moreover, a color filter 46, a transparent electrode 47a, and an alignment film 48a are disposed between the low refractive index layer 45 and the liquid crystal layer 49. The reflector 50 is disposed on the surface of the transparent substrate 44b that faces the liquid crystal layer 49, and a transparent electrode 47b and an alignment film 48b are disposed between the liquid crystal layer 49 and the reflector 50.
On the other hand, a phase retarder 43, a polarizer 42, and an optical path control layer 41 are disposed in this order on the opposite surface of the transparent substrate 44a from the liquid crystal layer. The optical path control layer 41 has a higher refractive index than the low refractive index layer 45 and is provided with slopes at an angle of 35 to 48°. In the liquid crystal display device of the second conventional example, the light introduced from the light source 51 into the transparent substrate 44a is propagated in the direction away from the light source 51 by repeating total reflection between the optical path control layer 41 and the transparent substrate 44a due to the refractive index difference between the transparent substrate 44a and the low refractive index layer 45 and the refractive index difference between the optical path control layer 41 and the air. Moreover, when the total reflection conditions are not fulfilled by the presence of the slopes in the optical path control layer 41, the light from the light source 51 is emitted from the transparent substrate 44a to the reflector 50, and then the light reflected by the reflector 50 is allowed to enter the liquid crystal layer 49 as illumination light.
As shown in FIG. 13, a liquid crystal display device of a third conventional example disclosed in JP 2002-121050 A includes a liquid crystal layer 69, a pair of transparent substrates 64a, 64b sandwiching the liquid crystal layer 69, and a light source 71 located opposite to the side of the transparent substrate 64a. Light from the light source 71 is introduced into the transparent substrate 64a directly and via a reflector. The liquid crystal display device of the third conventional example is a transmission-type liquid crystal display device in which an optical path control layer 61 is disposed on the side of the transparent substrate 64a that faces away from the liquid crystal layer, and light whose optical path has been changed by the optical path control layer 61 is allowed to enter the liquid crystal display element as illumination light.
The transparent substrates 64a, 64b are made of a transparent glass material. A transparent low refractive index layer 65 having a lower refractive index than the transparent substrate 64a is disposed on the surface of the transparent substrate 64a that faces the liquid crystal layer 69. Moreover, a color filter 66, a transparent electrode 67a, and an alignment film 68a are disposed between the low refractive index layer 65 and the liquid crystal layer 69. Further, a transparent electrode 67b and an alignment film 68b are disposed in this order on the surface of the transparent substrate 64b that faces the liquid crystal layer 69, and a polarizer 62b is disposed on the opposite surface of the transparent substrate 64b from the liquid crystal layer.
On the other hand, a polarizer 62a, the optical path control layer 61, and a reflector 70 are disposed in this order on the opposite surface of the transparent substrate 64a from the liquid crystal layer. The optical path control layer 61 has a higher refractive index than the low refractive index layer 65 and is provided with slopes at an angle of 35° to 48°. In the liquid crystal display device of the third conventional example, the light introduced from the light source 71 into the transparent substrate 64a is propagated in the direction away from the light source 71 by repeating total reflection between the transparent substrate 64a and the reflector 70 due to the refractive index difference between the transparent substrate 64a and the low refractive index layer 65, and the reflector 70. Moreover, when the total reflection conditions are not fulfilled by the presence of the slopes in the optical path control layer 61, the light from the light source 71 is allowed to enter the liquid crystal layer 69 as illumination light.